P. ALPERT and H. SHAFIR D. ISSAHARYadmin.indiaenvironmentportal.org.in/files/file/dead sea... ·...

RECENT CHANGES IN THE CLIMATE AT THE DEAD SEA –A PRELIMINARY STUDY

P. ALPERT� and H. SHAFIRDepartment of Geophysics and Planetary Sciences, Tel-Aviv University, Israel 69978

D. ISSAHARYDead Sea Works, Research Department, South Dead Sea, Israel

Abstract. In the last decade pan evaporation measured at the Southern Dead Sea has significantlyincreased. Wind, temperature and humidity measurements at the Dead Sea starting in the 1930s as wellas 3-D model simulations all seem to indicate a statistically significant change in the local climate ofthe Dead Sea region. The potential contribution to this climatic change through the weakening of thelocal land-sea breeze circulation caused by the reduction in the Dead Sea surface area in 1979–1981,is examined. It is suggested that since the breeze tempers the Dead Sea climate, its weakening hascaused the air temperature to increase, the relative humidity to decrease and thus increased the panevaporation. The climatic changes as implied by the MM4 Mesoscale PSU/NCAR model simulations,seem to fit the observed changes and to suggest a local tendency to the more arid climate that nowprevails to the south of the study region.

1. Introduction

The Dead Sea has attracted attention from ancient times as it does today; it islocated at the lowest spot on the earth; currently, about –410 m below sea level.It is a part of the Rift Valley, extending from the Taurus mountains in Turkey toEast Africa. Its waters are the densest and saltiest of any natural water body, andgave the Dead Sea its name, as almost no form of life higher than bacteria existsin this water, see Oren (1993). The potash factory, located on the southern part ofthe lake, makes use of the concentrated salts and the arid climate to manufacturepotash, bromine and other salts by natural evaporation. The arid climate provideshigh values of evaporation which make the production process successful.

It is this special Dead Sea climate which is the focus of this paper. The pio-neering studies of the Dead Sea climate were those of Ashbel, who made extensivemeasurements of meteorological as well as hydrological parameters starting in the1930s; these were summarised in Ashbel (1975). In recent years, Bitan (1974) hasinvestigated special features of the Dead Sea wind regimes.

Recently, Israel and Jordan have made extensive use of the water previouslyflowing into the Dead Sea. As a result, parts of the Dead Sea have dried. Stein-horn (1981), Klein (1982), Anati and Shasha (1989) and others investigated thesechanges in the area of the Dead Sea from different points of view. One result hasbeen the separation of the Northern basin of the Dead Sea

from the Southern basin which only continues to exist artificially as a watersurface with a depth of only 1–2 m, and is used as an evaporation pond for the

� Work partly done while on Sabbatical Leave at Data Assimilation Office, Code 910.3,NASA/GSFC, Greenbelt, MD20771.

Climatic Change 37: 513–537, 1997.c 1997 Kluwer Academic Publishers. Printed in the Netherlands.

514 P. ALPERT ET AL.

local mineral industries (Klein, 1985). The position of the region studies is shownin Figures 1 and 2.

The fact that the Dead Sea influences the climate of its neighboring regionswas discussed by Ashbel (1939). More recently Bitan (1977) investigated theseinfluences on the local wind regime. In the last decade evidence has emerged thatthe drying of the Dead Sea has caused a change in the climate of the Dead Sea basin.Stanhill (1994) calculated the effect of the Sea’s increasing salinity and temperaturein reducing the lake evaporation. Cohen and Stanhill (1996) have shown that thesolar irradiance and maximum temperature to the South Dead Sea have decreased,and the minimum temperature increased during the last 50 years.

In this paper, the climatic change at the Dead Sea caused by the drying ofthe Dead Sea is investigated. The recent rapid increase of the pan evaporationin Sdom (S in the figures) is described in Section 2. Section 3 suggests that thedrying of the lake may explain the recent local change in the Dead Sea climate.Section 4 discusses the significance of the variations in the lake breeze to the DeadSea climate change. Three-dimensional numerical modeling (PSU/NCAR versionMM4) is described in Section 5. The model was run with the extended area of theDead Sea in the past as well as with its present reduced area. Model results agreewith the evidence presented that a climate change has occurred in the Dead Sea.

2. The Change of Evaporation Climate to the South Dead Sea

2.1. THE PRESENT STATE OF THE DEAD SEA

The general region of study is presented in Figure 1, while the Dead Sea regionand measurement stations are shown in Figure 2. Part of the former Dead Seaseen in Figure 2 has dried; and the former Lisan Straits west of the former Lisanpeninsula is now dry land. The Northern basin is thus completely disconnectedfrom the Southern basin, which exists as an artificial lake, with water pumped fromthe Northern basin via a canal to the Southern basin to maintain a depth of notmore than 1–2 m (Klein, 1985). The local industries use the Southern basin asevaporation ponds.

2.2. PAN EVAPORATION CHANGES IN SOUTH DEAD SEA

In recent years pan evaporation measured at Sdom (31�020 N, 35�240 E, see stationdenoted by S in Figure 2) have shown a 12–15% increase (Figure 3). The pan is astandard U.S. Class A pan, 1.2 m diameter, depth of 25 cm. It is located within theevaporation ponds region and inside a fenced area of the Dead Sea Works, and isprotected to avoid animal or bird interference. The measurements are performed asfollows. Each day at 8 a.m. three micrometer readings of the pan level are taken andthe average reading is recorded. Evaporation from the pan is corrected to includethe small amounts of rainfall.

RECENT CHANGES IN THE CLIMATE AT THE DEAD SEA 515

Figure 1. Area of the study domain. Topography contours are dashed with 300 minterval. At the north westerly corner the general area (East Mediterranean) is shown along with theextended model domain with the six radiosonde locations employed in the model simulations.

Comparison of two pan measurements performed in parallel for several years inSdom indicates the measurements to have a variance of 2.6 mm2 d�2 and a standarddeviation of about 1.6 mm d�1. Since the variance is additive, the annual standarddeviation was estimated as 30.6 mm yr�1, less than 1% error in the annual valuesin Sdom.

In the 1960–1980 period, the annual totals of pan evaporation values were 3.40–3.60 m yr�1. In the last decade it increased to 3.80–4.00 m yr�1, and in 1994 itreached

a value above 4.00 m yr�1. Since 1981 values below 3.50 m yr�1 have not beenrecorded, while in earlier periods about half of the years exceeded this value; seeFigure 3.


Figure 2. Study domain of the Dead Sea. Topography contours are with 600 m interval. The areabetween the dotted lines is the drying zone. Letters KR, ZR, RR, CR and S stand for locations ofKidron River, Zeelim River, Rachaf River, Chemar River and Sdom, respectively.


Figure 3. Annual pan evaporation at Sdom (S) for the years 1962–1994. Line of 350 cm yr�1, wasadded.

The purpose of this paper is to explore the reasons for this increase. Two yearswith relatively low evaporation can be related to volcanic eruptions. The Agungeruption in March 1963 was a major one with a Dust Veil Index of 800 (Mass andPortman, 1989), and in this year a sharp decline in the evaporation was recordedat S (Figure 3). The Pinatubo eruption in June 1991 was also major (Song et al.,1996), and an evaporation decline was recorded in 1991 and also in 1992. 1992was also a year with heavy precipitation that may partly explain the evaporationreduction in that year. For instance, in Jerusalem an exceptionally high rainfall of1134 mm yr�1 was reported for 1992 as compared to the 30 y normal of 492 mmyr�1 (Israel Met. Serv., 1967).

The annual rainfall at Sdom is only 50 mm yr�1 and its effect on evaporation isin general small. Correlations between Sdom evaporation and Sdom or Jerusalemrainfall values for the period 1960/1–1994/5 were calculated and found, as expect-ed, negative but very low�, i.e., –0.11 and –0.08 respectively. Similarly, Cohen

� The indirect effect of rainfall on the evaporation through the change of the Dead Sea area and thelocal climate is apparently much stronger and is the main issue of the present study. Obviously, thisindirect relation of rainfall-evaporation involves other factors as well such as rainfall on the catchmentarea, river flow and water usage.


Figure 4. Average monthly evaporation at Sdom (S). Current period (1982–1993) compared withprevious period (1962–1981), and the monthly difference between the two periods.

and Stanhill (1996) have recently analyzed the variance explained by the rainfallinfluence over the annual maximum temperature variability employing a multiplelinear regression analysis in Sdom. They found that rainfall explains only 6% ascompared to 30% in other Jordan Valley locations in the North. They have alsofound negative relationships in all cases, i.e., temperature decreased with increasedrainfall.

Figure 4 presents mean pan evaporation per month in recent years (1982–1993),as compared to earlier years (1962–1981). The differences in the monthly valuescan also be seen at the bottom of the figure. The main increase is found in theSpring–early Summer months with a peak in May, while the winter contribution tothe change is insignificant.


Figure 5. Dead Sea MSL changes for 1966–1995 following Anati et al. (1987), Anati and Shasha(1989) and Anati (1997).

3. Local Dead Sea Climatic Change as Affected by the Drying Up of the Sea?

3.1. DRYING OF THE DEAD SEA

Figure 5 shows the recent changes in the mean sea level of the Dead Sea. Accordingto Anati and Shasha (1989) the average rate of the drop in the level of the DeadSea was –0.8 m yr�1 during 1981–1989. In 1969–1977, the average drop was lessthan half –0.39 m yr�1. The rapid drop of the Dead Sea MSL was associated with asharp decline in the Dead Sea surface area. The Dead Sea area was nearly constantat 940–950 km2 during 1940–1950 (Stanhill, 1994). In 1977–1979, when the MSLwas –402 m, the Northern basin became disconnected from the Southern basin andabout 50 km2 west to the Lisan Straits have dried out during a short period of time.In 1981, the area was only 796 km2 (Steinhorn, 1981), and the remaining Southernbasin has a depth of only 1–2 m. It is interesting to note that the pan evaporationincrease at Sdom began in 1981 following the major contraction of the Dead Sea.During 1980–1990, areas of the Northern basin dried, and the Dead Sea area in1987 was 686 km2 (Hydrological Year Book of Israel, 1987).


3.2. SIGNIFICANT CHANGES OF METEOROLOGICAL VARIABLES IN THE SOUTHDEAD SEA

Meteorological data at Sdom (S) from recent years was compared to those measuredpreviously by Ashbel (1975). The measuring station at S remained at nearly thesame location during these years. Three different climate stations operated in the1930s–1940s; in 1934, measurements were at the foot of the Salt mountain; in1936, at the evaporation ponds (S) and in 1942, near the Dead Sea Works Factory.Ashbel used the 1936 (evaporation pond) station and the present S measurementsare also at the evaporation ponds. The station that operated in 1942 is the onecurrently maintained by the Israel Meteorological Service.

Over 45,000 observations were considered for the relative humidity calculationin the 1935–1941 period, and over 20,000 observations in the 1985–1987 period.For the temperature calculations the corresponding numbers are over 35,000 andover 15,000. The same calculating procedure was applied in all cases. First, themonthly average from the hourly means was calculated, than the annual average.All available data were considered for the calculations. The average humidity inthe 1937–1942 period� was 55%, as compared to 46% in the recent 1985–1987period (Issahary et al., 1985, 1986, 1987). This 9% decline implies an evaporationincrease. The average temperature from Ashbel’s data (1975) was 25.7 �C whilethe present temperature is 26.3 �C. This 0.6 �C increase also implies an increase inpan evaporation.

In order to statistically examine the more recent changes, hourly meteorologicaldata at Sdom between 1975–1989 were studied. The meteorological parametersexamined were air dry and wet-bulb temperatures, wind speed, cloud amount andpan evaporation. Based on theT -test the significance of the changes in these hourlyparameters was determined by comparing the recent period of 1982–1989 with the1975–1981 period before pan evaporation increased.

The calculation procedure was as follows. The hourly data at 08, 14 and 20for each month, year and parameter were employed to calculate monthly averagesfor each hour. Then, the means for the first period (1975–1981) and the second(1982–1989) were computed. The statistical tests’ results are presented in Table I.The main conclusions were that wind speeds from March to July have statisticallydecreased; air temperatures from February to June have decreased, but increasedfor July

to September. The wet-bulb temperatures show a similar trend to that of thedry bulb temperatures. Finally, evaporation increased from April to September (seealso Figure 4).

� Ashbel (1975) used for humidity measurements the classical hair hygrograph while recentmeasurements were taken by the same method during 1985–1986 and in 1987 through the dry andwet-bulb temperatures, Issahary (1987). Although the instrumental changes are large, these changesare statistically significant higher than 99% and are in the same direction as our other findings to bepresented next.


Table ISignificant changes of average hourly meteorological parameters at Sdom. New period (1982–1989)against previous period (1975–1981). First line: + increase, – decrease of parameter in last period.Second line: The difference of parameter between two periods. Third line: The significant level ofthe climatic change – three stars (� � �): Significant level of more than 99%, two stars (��) – morethan 97.5%, one star (�) – more than 95%, without star – more than 90%. A blank box denotesinsignificant change


We have also examined with the same procedures the climatic trends in otherstations of the Jordan Valley. i.e., Tirat Zvi, Jericho and Eilat, Figure 1. Table IIsummarizes the significance of changes in the monthly averages of relative humid-ity, temperature, wind speed, atmospheric pressure and cloud amounts between therecent period (1982–1990) compared with the earlier one (1966–1981). The peri-ods averaged are not exactly corresponding to those of pan evaporation because ofdata availability. In contrast to the calculations from Sdom in Table I, where hourlydata were used, in Table II derivations of the monthly mean data were employedand a similar pattern to that found at Sdom in Table I was obtained. At Sdom (S), areduction in the May–July wind speed was found; an increase in July–Septembertemperature and a decrease in the relative humidity in May–June. These changeswere not found at the other three Jordan Valley stations. On the contrary, an increasein wind speed was found at Jericho and Eilat, and an increase of relative humidityin Tirat Zvi and Jericho. It should be noted that in other months there is also anincrease in temperature and decrease in relative humidity at Sdom but these are notstatistically significant. Another finding was increases in air pressure at Jericho andEilat for several months. Also, the seemingly highly significant decrease in windspeed at Tirat Zvi is certainly an instrumental error (J. Mishaelli, pers. comm.). Ahypothesis to explain these changes is presented next.

3.3. EXPLANATION TO THE INCREASE IN PAN EVAPORATION AT THE SOUTH DEADSEA

As a result of its decreasing area the Dead Sea breeze has weakened. This windnormally moderates the Dead Sea climate, and its weakening causes a decreasein relative humidity, and an increase in the air temperature, and consequentlypan evaporation to increase. These climatic changes are most marked during theSpring–early Summer months when the sea-land temperature differences are attheir maximum since the lake is still cold from the low winter temperatures, whilethe land has already warmed. Therefore, during these months a climatic changetoward a hotter and drier climate is expected as well as an increase in the panevaporation.

4. Role of the Dead Sea Breeze in Tempering the Climate of the Dead SeaRegion

4.1. THE DEAD SEA BREEZE AND ITS INFLUENCE ON TEMPERATURE

Figure 6a shows the diurnal variation for the normalized February–June meanwind speed at Sdom (S), during the recent period of 1987–1989 (measured by D.Issahary), and the past period of 1935–1941 (Ashbel, 1975). There are differencesin the absolute wind speed between the two curves probably due to the instru-mentation. Ashbel (1975) analysed measurements using a Dines anemograph that

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Table IISignificant changes of average monthly meteorological parameters in four stations at the Jordan Valley. New period (1982–1990)against previous period (1966–1981). RH – means Relative Humidity, T – Temperature, V – Wind Speed, P – Atmospheric Pressureand C – Cloud Amounts. First line: + increase, – decrease of parameter in last period. Second line: The difference of parameterbetween two periods. Third line: The significant level of the climatic change as in Table I. The single # signs missing data, while## signs data error


suffered from strong corrosion in the climate of the Dead Sea, Ashbel (1975),while recent measurements are based on cup anemometers. Therefore, Figure 6apresents the normalized wind speed at the two periods performed by subtractingthe daily means and dividing by the standard deviations in each of the two periods,respectively. Figure 6a suggests a climatic change in the local diurnal pattern of thelake breezes. During the two periods the wind speed increase from the morning tillnoon time is characteristic of the breeze (Alpert et al., 1982). In the earlier period,however, the wind speed began to rise as early as 7 a.m., intensified at 9 a.m., andreached its peak at 3 p.m. while more recently, the wind speed rise begins only ataround 10 a.m. and reaches its peak later at 4 p.m.. Hence, the diurnal wind speedamplitude which reflects the local sea breeze was much emphasized during the1930s–1940s compared to recent data. Also, in the recent wind speed variationsanother maximum at 8 p.m. exists. This maximum is not found in the 1935–1941averages and will be explained in the next section.

Figure 6b shows the air temperature at the Southern Dead Sea. The temperaturein the 1930s–1940s in the morning hours was somewhat lower than today andthe maximum daily temperature occurred one hour later. A possible change inthe recording time may be ruled out, since the phase changes in Figure 6a, b arenot in the same direction. The phase shift and the higher recent maximum can beexplained by the weakening and delaying of the lake breeze. In the past, this localbreeze cooled the air and moderated the temperature rise during the morning hours.Now, this effect is weaker, and a steep temperature rise occurs in the morning andthe temperature peaks earlier. It should be noted that each hourly data in Figure6a, b is the mean of about 600 measurements in the 1930–1940, and about 450measurements in the 1980–1990. However, given the short time series, particularlyfor the wind (Figure 6a), and the instrument changes, these findings should betreated with some caution.

4.2. SPECIAL OBSERVATIONAL PERIOD AT THE DEAD SEA

To further investigate the role of the Dead Sea breeze in affecting the local climate,special observations were conducted at the Dead Sea western coast during the10–13 May 1992, a season when the breeze is particularly strong. The wind vector,temperature, wet-bulb temperature, radiation and pan evaporation were measuredsimultaneously at six automatic stations whose locations are indicated in Figure 7.Measurements at stations KR and S were made every minute and averaged everyhour, while at stations RR and CR measurements were made every 20 s andaveraged every 10 min. For stations JR and J only the 08, 14 and 20 measurementswere available. Figure 7 is for noon (2 p.m.) of 12 May and for evening (8 p.m.) ofthe 10 May (dashed). At noon, the wind direction is north-westerly in Jerusalem,and north-easterly in Jericho. However, the wind direction in the Dead Sea stationsis from the Lake. In KR, RR, CR and S winds blow with a local lake breezecomponent. These effects were also noted by Bitan (1977).


Figure 6a.

At the evening of 10 May another phenomenon characteristic for the DeadSea climate was noticed. At this time, the Mediterranean breeze reaches the DeadSea in the Spring and Summer. The airmass associated with this wind falls about1200 m from the Judean Mountains, heats adiabatically, dries and penetrates theDead Sea region. A similar phenomenon further north at the Sea of Galilee wasalso simulated and discussed by Alpert et al. (1982). The wind directions in theevening are westerly to north-westerly with relatively high speeds of 5–10 knotsat all the stations (Figure 7, dashed). The hot and dry wind certainly affects theevaporation in Sdom. The adiabatic heating and drying of the Dead Sea regionincreases the evaporation (Ashbel, 1939). Hence, the changes in the interaction ofthe Mediterranean breeze with the Dead Sea breeze in the presence of the steeptopography may have therefore caused the increase in pan evaporation at Sdomin recent years. In the past, when the Dead Sea breeze was stronger, it delayed


Figure 6b.

Figure 6. Diurnal variations in (a) normalized wind speed, (b) temperature at Sdom for the February–June means. New 1987–1989 and older 1935–1941 periods, are shown.

the penetration of the Mediterranean breeze. Due to the recent weakening of theDead Sea breeze, the Mediterranean breeze penetrates strongly into the DeadSea region, and this hot and dry wind increases the evaporation. This happensduring both Spring and Summer when the Mediterranean breeze is significant. Thephenomenon is clearly seen in Figure 6a. A new peak in the wind speed is observedin the more recent years at 8 p.m., while in the past, when the Dead-Sea breezedelayed the Mediterranean breeze penetration, this peak is missing.

Figure 8 shows the diurnal temperature on the 12 May 1992. From 8 a.m. till3 p.m. the highest temperature was measured in Sdom, and the lower temperaturewas recorded on RR with medium values at CR. S is the farthest from the lakeof these stations, while RR is the closest. As suggested earlier, this temperaturecharacteristic is strongly related to the lake breeze, which cools the areas neigh-boring the Northern basin but is weaker further to the south. In the next section ourhypothesis will be examined with the aid of mesoscale model simulations.


Figure 7. Surface wind vectors for 12 May 1992 12 UTC (solid) and for 10 May 18 UTC (dashed).Arrows indicate wind direction, and line on top of the arrow its speed: diagonal line – 3 knots, shortline – 5 knots, long line – 10 knots. Open circles denote fair weather. Letters JR, J, KR, RR, CR andS stand for Jerusalem, Jericho, Kidron River, Rachaf River, Chemar River and Sdom, respectively.


Figure 8. Diurnal temperature variations at RR, CR and S on 12 May 1992.

5. Mesoscale Modeling

5.1. THE MODEL AND BACKGROUND DETAILS

A three-dimensional model was run in order to investigate the proposed hypothesisfor the climatic change in the Dead Sea region. The NCAR/PSU model versionMM4 (Anthes et al., 1987) was run for 24 h for a late Spring-early Summer event on17 June, 1987, at time when a very pronounced local change was noticed. For thesurface-evaporation fluxes calculations the model uses the explicit, high resolutionPBL model (Zhang and Anthes, 1982) and the fluxes formulation follows Carlsonand Boland (1978). The model domain was 360� 360 km; grid size of 5 km, andwith 16 vertical levels at the altitudes of 1000 hPa, 991, 982, 964, 937, 901, 856,802, 730, 640, 550, 460, 370, 280, 190 and 100 hPa. A time step of 10 sec was usedand for the initialization six soundings from the area were employed (Figure 1).This initialization and other aspects of our high-resolution mesoscale simulations


Table IIIDescription of land-use categories and physical parameters used in the 3-D model simulations

No. Land-use Albedo Moisture Emissivity Roughness Thermal inertiadescription (%) avail. (%) (% at 9 �m) length (cm) (cal cm�2 K�1 s�0:5)

1 Water 8 100 98 0.0001 0.062 Wet land 14 50 95 20 0.063 Desert 25 2 85 10 0.02

over the region can be found in Lieman and Alpert (1993). Since the evaporationof the Dead Sea water is about half that of pure water (Stanhill, 1994), moistureavailability for the Dead Sea grid points was assumed 0.5. Over the Dead Sea,47 points exist in the model domain, 34 of which over the Northern basin and 13over the Southern basin. The latter includes the evaporation ponds of the Dead SeaWorks. Three runs were performed; the first run with the Dead Sea area as it wasbefore 1979. In the second run the Dead Sea was omitted, and all its grid pointsare in the state of a ‘desert’. The third run represented the Dead Sea at its presentconfiguration. In the latter, the Northern basin exists as a full sea (neglecting thedecrease in its area as well), and the Southern basin is partly dry in the followingmanner. Two grid points west to the Lisan Straits are dry land – ‘desert’, and therest assumed ‘wet land’. This reflects the state of the Dead Sea in the last decade,in which part of the Southern basin is dry, and the other part exists artificially.The second part is represented by ‘wet land’ of the land use category although thephysical characteristics of the ponds are somewhat different than the ‘wet land’. Forthe different categories of land-use parameters used in the model for the Summersee Table III (from Anthes et al.,

1987). The roughness length may not fit well into these land-use but these effectswere neglected. Also, the Dead Sea water temperature was assumed constant at30 �C, the mean June value (Ashbel, 1975) ignoring the daily variation of about5 �C (Ashbel, 1975). However, over the Southern Dead Sea ‘wet-land’ grid pointsin the model were allowed to change the ‘water’ temperatures. The Mediterraneansea surface temperature was assumed constant at 23.5 �C. Other important effectsignored in the present runs are the effect of the increase of the Dead Sea SST inthe last 50 years as reported by Stanhill (1990), and the reduction of the globalirradiance at Sdom as reported by Cohen and Stanhill (1996). Here the focus wassolely to estimate the potential contribution of the changing area of the Dead Sea.

5.2. SIMULATED CHANGES OF THE DIURNAL METEOROLOGICAL FIELDS AT THESOUTH DEAD SEA

Simulated diurnal variations of wind speed (a), temperature (b), relative humidity(c) and pan evaporation (d) on 17 June 1987 at Sdom for the three runs are shownin Figure 9a–d. The three runs were as follows. First, the ‘full sea’ – the Dead Sea


Figure 9. Model simulated diurnal changes in the meteorological fields at Sdom on 17 June 1987.The ‘full sea’ run – full line; ‘no sea’ – dashed-dotted line; ‘present sea’ – dotted line; differencefields, i.e., ‘full sea’ minus ‘present sea’ – dashed line at the bottom. (a) wind speed (m s�1), (b)temperature (�C), (c) relative humidity (%), (d) pan evaporation flux (mm s�1 � 10�4).

as it used to be in the past, the ‘present sea’ – present state of the sea and ‘no sea’.At the bottom of each figure the differences between ‘present sea’ and ‘full sea’ areshown. These differences represent the model simulated climatic changes inducedby the area shrinking of the Dead Sea.

The wind speed, depicted in Figure 9a, shows two peaks, at 10 a.m. and at 4 p.m.The first peak is related to the Dead Sea sea-breeze, while the second refers to the


penetration of the Mediterranean Sea breeze. The strongest daily wind speed is forthe ‘full sea’ run, while the lowest speed is for the ‘no sea’ run. The ‘present sea’run shows the intermediate values. The wind direction during the day is easterly,reflecting the local lake breeze. It is missing when the sea is absent and partlyexists in the present state of the sea. The mean sea breeze is 4–5 m s�1 for the‘full sea’ run, 3.5–4.5 m s�1 for the ‘present sea’ and 3–4 m s�1 for ‘no sea’.The weakening of the breeze is shown at the base of the figure. The difference inwind speed between ‘full sea’ and ‘present sea’ is up to 0.8 m s�1 at 11 a.m. andat 3 p.m. At 4 p.m. the Mediterranean breeze penetrates the region, and the windspeed reaches 5 m s�1 with a north-westerly direction.

As expected, the daily temperature is highest in the ‘no sea’ run (Figure 9b). With‘full sea’ the temperature is lowest, and again the ‘present sea’ has intermediatevalues. During the night the picture is opposite and the ‘full sea’ state has maximumvalues. The temperature reached its peak at 4 p.m., 36.8 �C without the sea, 35.1 �Cfor the ‘full sea’ and 35.3 �C for the present sea. The Dead Sea clearly tempersthe temperature in its vicinity. During the day it lowers and during night it raisesthe neighborhood’s temperature, see the difference line. The difference reaches amaximum of 0.9 �C at 1 p.m.

The relative humidity (Figure 9c), is high during the night and low during thedaytime because of its reversed relationship with the temperature. The relativehumidity is highest for ‘full sea’, lowest for ‘no sea’, and has intermediate valuesfor ‘present sea’. The influence of the sea on the relative humidity of its neighboringareas is twofold; first, a water source increases the relative humidity in its vicinity,and secondly, the cooling of the air causes an increase in relative humidity. In thepresent state of the sea, the relative humidity is less than in the past because ofthe sea’s drying. The difference between the runs reaches 7% at 7 a.m., with thenegative difference noticed throughout the day.

Figure 9d shows the calculated pan evaporation based on the model simulations,i.e. assuming moisture availability of 1 or doubling the model results calculatedfor moisture availability of 0.5. The maximum pan evaporation is for ‘no sea’,while the evaporation is minimal for ‘full sea’. The difference between the pastand present state runs reaches a value of 5:5 � 10�4 mm s�1 at noon. The totaldaily pan evaporations at the three runs were computed as follows. First, the meanevaporation fluxes of the day were found. Next, the daily evaporations were foundand multiplied by the ratio between the moisture availability of pure water (100%),and that of the grid point under investigation in every run. Hence, the total dailypan evaporations at S was found to be 5.9 mm d�1 for ‘full sea’, 27.4 mm d�1 for‘no sea’ and 14.4 mm d�1 for the ‘present state’. These estimates may be comparedwith the actual value of 18.5 mm d�1 measured on 17 June 1987. It should be notedthat the model seems to predict quite high values for the ‘no sea’ case as comparedto the Jordan Valley values, but this may be due to the even drier conditions overthe hypothetically totally dry Dead Sea.


Figure 10. Model-simulated surface winds (m s�1) on 17 June 1987 12 UTC (14 LT). Scale of thewind speed is at the bottom. Topography is dashed with 300 m interval. Letters JR, J and S stand forJerusalem, Jericho and Sdomrespectively. (a) ‘present sea’ (b) the difference, i.e. ‘full sea’ minus ‘present sea’.

5.3. AREAL DISTRIBUTIONS OF THE METEOROLOGICAL FIELDS

Figures 10–13 show the geographical distributions of the wind vector, temperature,relative humidity and lake evaporation respectively at 12 UTC (1400 LST or 1500with Summer time). In each of the figures (a) is for the present state, and (b) isthe difference map between ‘present state’ and ‘full sea’ (‘full sea’ minus ‘presentsea’). Hence, (b) represents the simulated climatic change.

Figure 10a, b shows the wind vectors. The Mediterranean breeze does not pen-etrate the Dead Sea region at noon, while close to the Dead Sea the easterly lakebreeze dominates (Figure 10a). From the difference map (Figure 10b) (‘full sea’minus ‘present sea’) the influence of the Dead Sea drying on the local breeze can beseen. The difference is especially apparent in the Southern basin where the change


Figure 11. As Figure 10 but for the surface temperature (�C). Here, the difference is ‘present sea’minus ‘full sea’. Temperature interval is 2 �C in (a) and 1 �C in (b).

in land-use took place. The difference wind vectors in the Southern basin representthe weakening of the Dead Sea breeze.

Figure 11a, b is for air temperatures at 12 UTC. The difference map (‘presentsea’ minus ‘full sea’ in this case) shows a difference of 1 �C in the Southern basin,representing the simulated climatic change in temperature.

Figure 12a, b is for relative humidity. A high value of 45% over the lake isnoticed in Figure 12a, compared to 20–25% over the land. Figure 12b shows

a difference of –4% between the two runs, representing the simulated effect ofthe climatic change on the relative humidity.

Figure 13a, b presents the lake evaporation maps. As already mentioned, thesalinity effect of the Dead Sea was taken into account by reducing the moistureavailability to 0.5. A high value of evaporation flux of 13:5� 10�5 mm s�1 for theSouthern basin can be seen in Figure 13a. The increase in lake evaporation reaches


Figure 12. As Figure 11 but for the relative humidity (%). The interval is 5% in (a) and 2% in (b).

a value of 10 � 10�5 mm s�1 at noon over the Southern basin, with a 7 � 10�5

mm s�1 increase in S (Figure 13b).

6. Discussion

Morton (1983) suggested a similar hypothesis to the one adopted here. Based onhis model results, he proposed that an inverse relationship exists between actual(areal) and potential (pan) evaporation. He has explained this by a micro-climatemodification. Two main processes seem to act while the Dead Sea dries. Bothprocesses tend to increase the potential evaporation. The first process was describedin this study and similarly in Morton (1983). In our study, it is the modification ofthe micro-climate through dynamical changes in the Mesoscale circulations. Theweakening of the local sea-land breeze causes an increase in air temperature, adecrease in relative humidity, and thus, an increase in the pan evaporation.


Figure 13. As Figure 11 but for the lake evaporation (mm s�1 � 10�5). Evaporation flux interval is1� 10�5 mm s�1 in (a) and 2� 10�5 mm s�1 in (b).

The second process is the increase in salinity of the lake. Stanhill (1994) sug-gested that decrease in the evaporation from the Dead Sea was accompanied by theincreased salinity from the Sea. Obviously, the reduction of the Dead Sea area wasassociated with increased salinity. The reduced evaporation from the Sea dimin-ishes the relative humidity in the air, which in turn may further increase the panevaporation at the Southern Dead Sea. It will be helpful to run the model with thesalinity changes as well. It can be done by reducing in the model, the Sea moistureavailability at the Southern Dead Sea. Further research on the salinity variationspotential effect on the climatic change is beyond the scope of this preliminarystudy.


7. Summary

Three-dimensional model simulations in combination with measurements and sta-tistical calculations strongly suggest that a climate change has occurred at theSouthern basin of the Dead Sea. The evidence presented here associates the DeadSea breeze weakening with the drying of the Dead Sea and reduction in its area.The breeze, which tempers the climate of the Dead Sea, becomes less vigorous,especially in the Spring early-Summer months when it probably causes increases inthe air temperature and decreases in the relative humidity, causing an increase in thepan evaporation at the South Dead Sea. This hypothesis indicates a desertificationtendency in the Dead Sea climate.

Acknowledgements

This work was partly performed while the first author (PA) held a National ResearchCouncil – NASA/GSFC Research Associateship. We wish to express our gratitudeto Marina Tsidulko, Ronen Liman and Yuval Shay-El for their assistance in runningthe 3-D model and drawing the maps. Thanks to Dr. Ayal Anis for his helpfulcomments, and to the Israel Meteorological Service for parts of the data. We thankalso Dr. Noah Wolfson and Dr. Zipora Klein for their helpful comments, and Dr.Alex Manes and Prof. G. Stanhill for their help in obtaining Dead Sea data. Thanksto Dr. Anati for data of the Dead sea MSL changes. We thank the reviewers fortheir very helpful comments and particularly to Rev. A for his extensive and mostvaluable comments.

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(Received 24 April 1995; in revised form 23 January 1997)

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